Neuroanatomical studies of sexual differentiation in the zebra finch song system have revealed two classes of effects which gonadal hormones exert upon the development of their target neurons. First, both androgens and estrogens exert a similar pleiotrophic effect upon the anatomical differentiation of their respective target neurons. Second, estrogen organizes the competence of its target neurons to respond to the activational effects of hormones in the adult. A consequence of estradiol-induced sexual differentiation in the telencephalic song system nuclei may be the induction of androgen receptor and the suppression of its own receptor.